The star operation on an entity \(e\)  of dimension \(k\) , denoted \(\star e\) ,
returns the union of the interiors of all entities of dimension at least \(k\)  that
recursively contain \(e\)  as a sub-entity [1]}[2]}.
For example, the star of a cell is simply its interior, since there are no
entities of higher dimension. The star of an internal face \(\star f\)  returns the
patch of cells formed of the two cells that share \(f\) , excluding the
boundary of the patch. Similarly, the star of a vertex \(\star v\)  returns the
union of the interiors of all edges, faces, and cells sharing \(v\) , as well as
the vertex itself; geometrically, this forms the patch of cells sharing \(v\) ,
again excluding the boundary of the patch. The stars of a vertex, edge, and
face are shown in fig:stars.
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